parasit ology
TRANSCRIPT
Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the
scope of parasitology is not determined by the organism or environment in question, but by their way of life. This
means it forms a synthesis of other disciplines, and draws on techniques from fields such as cell
biology, bioinformatics, biochemistry, molecular biology, immunology, genetics, evolution and ecology.
Fields
The study of these diverse organisms means that the subject is often broken up into simpler, more focused units,
which use common techniques, even if they are not studying the same organisms or diseases. Much research in
parasitology falls somewhere between two or more of these definitions. In general, the study of prokaryotes falls
under the field of bacteriology rather than parasitology.
[edit]Medical parasitology
See also: Human parasites
One of the largest fields in parasitology, medical parasitology is the subject which deals with the parasites that infect
humans, the diseases caused by them, clinical picture and the response generated by humans against them. It is
also concerned with the various methods of their diagnosis, treatment and finally their prevention & control. A parasite
is an organism that live on or within another organism called the host . These include organisms such as:
Plasmodium spp., the protozoan parasite which causes malaria. The four species of malaria parasites infective
to humans are Plasmodium falciparum,Plasmodium malariae, Plasmodium vivax& Plasmodium ovale.
Leishmania donovani, the unicellular organism which causes leishmaniasis
Entamoeba and Giardia, which cause intestinal infections (dysentery and diarrhoea)
Multicellular organisms and worms such as Schistosoma spp., Wuchereria bancrofti, Necator
americanus (hookworm) and Taenia spp. (tapeworm)
Ectoparasites such as ticks, scabies and lice
Medical parasitology can involve drug development, epidemiological studies and study of zoonoses.
[edit]Veterinary parasitology
Main article: Veterinary parasitology
The study of parasites that cause economic losses in agriculture or aquaculture operations, or which
infect companion animals. Examples of species studied are:
Lucilia sericata , a blowfly, which lays eggs on the skins of farm animals. The maggots hatch and burrow into the
flesh, distressing the animal and causing economic loss to the farmer
Otodectes cynotis, the cat ear mite, responsible for Canker.
Gyrodactylus salaris, a monogenean parasite of salmon, which can wipe out populations which are not resistant.
[edit]Structural parasitology
Main article: Structural parasitology
This is the study of structures of proteins from parasites. Determination of parasitic protein structures may help to
better understand how these proteins function differently from homologous proteins in humans. In addition, protein
structures may inform the process of drug discovery.
[edit]Quantitative parasitology
Main article: Quantitative parasitology
Parasites exhibit an aggregated distribution among host individuals, thus the majority of parasites live in the minority
of hosts. This feature forces parasitologists to use advanced biostatistical methodologies.
[edit]Parasite ecology
Parasites can provide information about host population ecology. In fisheries biology, for example, parasite
communities can be used to distinguish distinct populations of the same fish species co-inhabiting a region.
Additionally, parasites possess a variety of specialized traits and life-history strategies that enable them to colonize
hosts. Understanding these aspects of parasite ecology, of interest in their own right, can illuminate parasite-
avoidance strategies employed by hosts
[edit]Conservation biology of parasites
Main article: Conservation biology of parasites
Conservation biology is concerned with the protection and preservation of vulnerable species, including parasites. A
large proportion of parasite species are threatened by extinction, partly due to efforts to eradicate parasites which
infect humans or domestic animals, or damage human economy, but also caused by the decline or fragmentation of
host populations and the extinction of host species.
[edit]Taxonomy and phylogenetics
The huge diversity between parasitic organisms creates a challenge for biologists who wish to describe and
catalogue them. Recent developments in using DNA to identify separate species and to investigate the relationship
between groups at various taxonomic scales has been enormously useful to parasitologists, as many parasites are
highly degenerate, disguising relationships between species.
UNIT ONE
GENERAL PARASITOLOGY
LEARNING OBJECTIVES
At the end of this section the student is expected to:
• Discuss the various types of parasites and hosts.
• Explain the relationship between a parasite and the host and their effects.
• Discuss in detail the classification of medically important parasites.
• Explain the difference between the Cestodes, Nematodes, Trematodes and
protozoa
INTRODUCTION
Man and other living things on earth live in an entangling relationship with each other.
They don’t exist in an isolated fashion. They are interdependent; each forms a strand in
the web of life. Medical parasitology is the science that deals with organisms living in
the human body (the host) and the medical significance of this host-parasite
relationship.
ASSOCIATION BETWEEN PARASITE AND HOST
A parasite is a living organism, which takes its nourishment and other needs from a
host; the host is an organism which supports the parasite. The parasites included in
medical parasitology are protozoa, helminthes, and some arthropods. (See box 1 for
broader classification of parasites). The hosts vary depending on whether they harbor
the various stages in parasitic development. (See box 2) 2
BOX 1. DIFFERENT KINDS OF PARASITES
• Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g.
lice, ticks, mites etc.
• Endoparasites – parasites that live inside the body of their host, e.g. Entamoeba
histolytica.
• Obligate Parasite - This parasite is completely dependent on the host during a
segment or all of its life cycle, e.g. Plasmodium spp.
• Facultative parasite – an organism that exhibits both parasitic and non-parasitic
modes of living and hence does not absolutely depend on the parasitic way of
life, but is capable of adapting to it if placed on a host. E.g. Naegleria fowleri
• Accidental parasite – when a parasite attacks an unnatural host and survives. E.g.
Hymenolepis diminuta (rat tapeworm).
• Erratic parasite - is one that wanders in to an organ in which it is not usually found.
E.g. Entamoeba histolytica in the liver or lung of humans.
Most of the parasites which live in/on the body of the host do not cause disease
(non-pathogenic parasites). In Medical parasitology we will focus on most of the disease
causing (pathogenic) parasites. However, understanding parasites which do not
ordinarily produce disease in healthy (immunocompetent) individuals but do cause
illness in individuals with impaired defense mechanism (opportunistic parasites) is
becoming of paramount importance because of the increasing prevalence of HIV/AIDS
in our country. 3
BOX 2. DIFFERENT KINDS OF HOSTS
• Definitive host – a host that harbors a parasite in the adult stage or where the parasite
undergoes a sexual method of reproduction.
• Intermediate host - harbors the larval stages of the parasite or an asexual cycle of
development takes place. In some cases, larval development is completed in two
different intermediate hosts, referred to as first and second intermediate hosts.
• Paratenic host – a host that serves as a temporary refuge and vehicle for reaching an
obligatory host, usually the definitive host, i.e. it is not necessary for the completion of
the parasites life cycle.
• Reservoir host – a host that makes the parasite available for the transmission to
another host and is usually not affected by the infection.
• Natural host – a host that is naturally infected with certain species of parasite.
• Accidental host – a host that is under normal circumstances not infected with the
parasite.
There is a dynamic equilibrium which exists in the interaction of organisms. Any
organism that spends a portion or all of its life cycle intimately associated with another
organism of a different species is considered as Symbiont (symbiote) and this
relationship is called symbiosis (symbiotic relationships).
The following are the three common symbiotic relationships between two organisms:
Mutualism - an association in which both partners are metabolically dependent upon
each other and one cannot live without the help of the other; however, none of the
partners suffers any harm from the association. One classic example is the relationship
between certain species of flagellated protozoa living in the gut of termites. The
protozoa, which depend entirely on a carbohydrate diet, acquire their nutrients from
termites. In return they are capable of synthesizing and secreting cellulases; the
cellulose digesting enzymes, which are utilized by termites in their digestion.4
Commensalism - an association in which the commensal takes the benefit without
causing injury to the host. E.g. Most of the normal floras of the humans’ body can be
considered as commensals.
Parasitism - an association where one of the partners is harmed and the other lives at
the expense of the other. E.g. Worms like Ascaris lumbricoides reside in the
gastrointestinal tract of man, and feed on important items of intestinal food causing
various illnesses.
Once we are clear about the different types of associations between hosts and
parasites, we can see the effect the parasite brings to the host and the reactions which
develop in the host’s body due to parasitic invasion.
EFFECT OF PARASITES ON THE HOST
The damage which pathogenic parasites produce in the tissues of the host may be
described in the following two ways;
(a) Direct effects of the parasite on the host
• Mechanical injury - may be inflicted by a parasite by means of pressure as it grows
larger, e.g. Hydatid cyst causes blockage of ducts such as blood vessels
producing infraction.
• Deleterious effect of toxic substances- in Plasmodium falciparum production of
toxic substances may cause rigors and other symptoms.
• Deprivation of nutrients, fluids and metabolites -parasite may produce disease by
competing with the host for nutrients.
(b) Indirect effects of the parasite on the host:
Immunological reaction: Tissue damage may be caused by immunological
response of the host, e.g. nephritic syndrome following Plasmodium infections.
Excessive proliferation of certain tissues due to invasion by some parasites can
also cause tissue damage in man, e.g. fibrosis of liver after deposition of the ova of
Schistosoma. 5
BASIC CONCEPTS IN MEDICAL PARASITOLOGY
In medical parasitology, each of the medically important parasites are discussed under
the standard subheadings of morphology, geographical distribution, means of infection,
life cycle, host/parasite relationship, pathology and clinical manifestations of infection,
laboratory diagnosis, treatment and preventive/control measures of parasites. In the
subsequent section some of these criteria are briefly presented.
Morphology - includes size, shape, color and position of different organelles in different
parasites at various stages of their development. This is especially important in
laboratory diagnosis which helps to identify the different stages of development and
differentiate between pathogenic and commensal organisms. For example,
Entamoeba histolytica and Entamoeba coli.
Geographical distribution - Even though revolutionary advances in transportation has
made geographical isolation no longer a protection against many of the parasitic
diseases, many of them are still found in abundance in the tropics. Distribution of
parasites depends upon:
a. The presence and food habits of a suitable host:
• Host specificity, for example, Ancylostoma duodenale requires man as a host
where Ancylostoma caninum requires a dog.
• Food habits, e.g. consumption of raw or undercooked meat or vegetables
predisposes to Taeniasis
b. Easy escape of the parasite from the host- the different developmental stages of a
parasite which are released from the body along with faeces and urine are widely
distributed in many parts of the world as compared to those parasites which
require a vector or direct body fluid contact for transmission.
c. Environmental conditions favoring survival outside the body of the host, i.e.
temperature, the presence of water, humidity etc.
d. The presence of an appropriate vector or intermediate host – parasites that do not
require an intermediate host (vector) for transmission are more widely distributed
than those that do require vectors. 6
Once we are clear about the geographical distribution and conditions favoring survival in
relation to different parasites, effective preventive and control measures can more easily
be devised and implemented.
Life cycle of parasites - the route followed by a parasite from the time of entry to the
host to exit, including the extracorporeal (outside the host) life. It can either be simple,
when only one host is involved, or complex, involving one or more intermediate hosts. A
parasite’s life cycle consists of two common phases one phase involves the route a
parasite follows inside the body. This information provides an understanding of the
symptomatology and pathology of the parasite. In addition the method of diagnosis and
selection of appropriate medication may also be determined. The other phase, the route
a parasite follows outside of the body, provides crucial information pertinent to
epidemiology, prevention, and control.
Host parasite relationship - infection is the result of entry and development within the
body of any injurious organism regardless of its size. Once the infecting organism is
introduced into the body of the host, it reacts in different ways and this could result in:
a. Carrier state - a perfect host parasite relationship where tissue destruction by a
parasite is balanced with the host’s tissue repair. At this point the parasite and
the host live harmoniously, i.e. they are at equilibrium.
b. Disease state - this is due to an imperfect host parasite relationship where the
parasite dominates the upper hand. It can result either from lower resistance of
the host or a higher pathogenecity of the parasite.
c. Parasite destruction – occurs when the host takes the upper hand.
Laboratory diagnosis – depending on the nature of the parasitic infections, the
following specimens are selected for laboratory diagnosis:
a) Blood – in those parasitic infections where the parasite itself in any stage of its
development circulates in the blood stream, examination of blood film forms one
of the main procedures for specific diagnosis. For example, in malaria the
parasites are found inside the red blood cells. In Bancroftian and Malayan
filariasis, microfilariae are found in the blood plasma. 7
b) Stool – examination of the stool forms an important part in the diagnosis of
intestinal parasitic infections and also for those helminthic parasites that localize
in the biliary tract and discharge their eggs into the intestine.
In protozoan infections, either trophozoites or cystic forms may be detected; the
former during the active phase and the latter during the chronic phase. Example,
Amoebiasis, Giardiasis, etc.
In the case of helmithic infections, the adult worms, their eggs, or larvae are
found in the stool.
c) Urine – when the parasite localizes in the urinary tract, examination of the urine
will be of help in establishing the parasitological diagnosis. For example in
urinary Schistosomiasis, eggs of Schistosoma haematobium are found in the
urine. In cases of chyluria caused by Wuchereria bancrofti, microfilariae are
found in the urine.
d) Sputum – examination of the sputum is useful in the following:
• In cases where the habitat of the parasite is in the respiratory tract, as in
Paragonimiasis, the eggs of Paragonimus westermani are found.
• In amoebic abscess of lung or in the case of amoebic liver abscess bursting
into the lungs, the trophozoites of E. histolytica are detected in the sputum.
e) Biopsy material - varies with different parasitic infections. For example spleen
punctures in cases of kala-azar, muscle biopsy in cases of Cysticercosis,
Trichinelliasis, and Chagas’ disease, Skin snip for Onchocerciasis.
f) Urethral or vaginal discharge – for Trichomonas vaginalis
Indirect evidences – changes indicative of intestinal parasitic infections are:
a. Cytological changes in the blood – eosiniphilia often gives an indication
of tissue invasion by helminthes, a reduction in white blood cell count is
an indication of kala-azar, and anemia is a feature of hookworm
infestation and malaria.
b. Serological tests – are carried out only in laboratories where special
antigens are available. 8
Treatment – many parasitic infections can be cured by specific chemotherapy. The
greatest advances have been made in the treatment of protozoal diseases.
For the treatment of intestinal helminthiasis, drugs are given orally for direct action on
the helminthes. To obtain maximum parasiticidal effect, it is desirable that the drugs
administered should not be absorbed and the drugs should also have minimum toxic
effect on the host.
Prevention and control - measures may be taken against every parasite infectiving
humans. Preventive measures designed to break the transmission cycle are crucial to
successful parasitic eradication. Such measures include:
� Reduction of the source of infection- the parasite is attacked within the host, thereby
preventing the dissemination of the infecting agent. Therefore, a prompt diagnosis
and treatment of parasitic diseases is an important component in the prevention of
dissemination.
� Sanitary control of drinking water and food.
� Proper waste disposal – through establishing safe sewage systems, use of screened
latrines, and treatment of night soil.
� The use of insecticides and other chemicals used to control the vector population.
� Protective clothing that would prevent vectors from resting in the surface of the body
and inoculate pathogens during their blood meal.
� Good personal hygiene.
� Avoidance of unprotected sexual practices.
CLASSIFICATION OF MEDICAL PARASITOLOGY
Parasites of medical importance come under the kingdom called protista and animalia.
Protista includes the microscopic single-celled eukaroytes known as protozoa. In
contrast, helminthes are macroscopic, multicellular worms possessing welldifferentiated tissues and complex organs belonging to the kingdom animalia. Medical
Parasitology is generally classified into:
• Medical Protozoology - Deals with the study of medically important protozoa. 9
• Medical Helminthology - Deals with the study of helminthes (worms) that affect
man.
• Medical Entomology - Deals with the study of arthropods which cause or transmit
disease to man.
Describing animal parasites follow certain rules of zoological nomenclature and each
phylum may be further subdivided as follows:
Super class Super family
Phylum Subphylum Class Order Family Genus Species
Subclass Subfamily 10
FIGURE 1. CLASSIFICATION OF MEDICALLY IMPORTANT PARASITES
PROTOZOA METAZOA (HELIMINTHS)
Sarcodina (Amoebae): Platyhelminthes:
(a) Genus, Entameba: Trematodea:
E.g. Entameba histolytica (a) Genus Schistosoma
(b) Genus Endolimax E.g. S. mansoni
E.g. Endolimax nana (b) Genus Fasciola
(c) Genus Iodameba E.g. F. hepatica
E.g. Iodameba butchlii Cestoda:
(d) Genus Dientmeba (a) Genus Diphylobotrium
E.g. Dientameba fragilis E.g. D. latum
Mastigophora (Flagellates): (b) Genus Taenia
(a) Genus Giardia E.g. T. saginata
E.g. G. lamblia (c) Genus Echinococcus
(b) Genus Trichomonas E.g. E. granulosus
E.g. T. vaginalis (d) Genus Hymenolepsis
(c) Genus Trypanosoma E.g. H. nana
E.g. T. brucci Nemathelminthes:
(d) Genus Leishmania (a) Intestinal Nematodes
E.g. L. donovani E.g. A. lumbricoides
Sporozoa (b) Somatic Nematodes
(1) Genus Plasmodium E.g. W. bancrofti
E.g. P. falciparum
(2) Genus Toxoplasma
E.g. T. gondi
(3) Genus Cryptosporidum
E.g. C. parvum
(4) Genus Isospora E.g. I. beli
Ciliates
E.g. Balantidium coli Parasitology
11
GENERAL CHARACTERISTICS OF MEDICALLY IMPORTANT PARASITES
Medically important protozoa, helminthes, and arthropods, which are identified as
causes and propagators of disease have the following general features. These
features also differ among parasites in a specific category.
(1) PROTOZOA
Protozoan parasites consist of a single "cell-like unit" which is morphologically and
functionally complete and can perform all functions of life. They are made up of a
mass of protoplasm differentiated into cytoplasm and nucleoplasm. The cytoplasm
consists of an outer layer of hyaline ectoplasm and an inner voluminous granular
endoplasm. The ectoplasm functions in protection, locomotion, and ingestion of food,
excretion, and respiration. In the cytoplasm there are different vacuoles responsible
for storage of food, digestion and excretion of waste products. The nucleus also
functions in reproduction and maintaining life.
The protozoal parasite possesses the property of being transformed from an active
(trophozoite) to an inactive stage, losing its power of motility and enclosing itself within
a tough wall. The protoplasmic body thus formed is known as a cyst. At this stage the
parasite loses its power to grow and multiply. The cyst is the resistant stage of the
parasite and is also infective to the human host.
Reproduction – the methods of reproduction or multiplication among the parasitic
protozoa are of the following types:
1. Asexual multiplication:
(a) Simple binary fission – in this process, after division of all the structures, the
individual parasite divides either longitudinally or transversely into two more
or less equal parts.
(b) Multiple fission or schizogony – in this process more than two individuals
are produced, e.g. asexual reproduction in Plasmodia. 12
2. Sexual reproduction:
(a) Conjugation – in this process, a temporary union of two individuals occurs
during which time interchange of nuclear material takes place. Later on, the
two individuals separate.
(b) Syngamy – in this process, sexually differentiated cells, called gametes,
unite permanently and a complete fusion of the nuclear material takes
place. The resulting product is then known as a zygote.
Protozoa are divided into four types classified based on their organs of locomotion.
These classifications are: amoebas, ciliates, flagellates, and sporozoans.
TABLE 1. CLASSIFICATION OF THE PATHOGENIC PROTOZOA:
PROTOZOA ORGAN OF IMPORTANT HUMAN
LOCOMOTION PATHOGENS
1. Rhizopoda Pseudopodia Entamoeba histolytica
(Amoeba)
2. Mastigophora Flagella Trypanosomes
(Flagellates) Leishmania
Trichomonas
Giardia
3. Sporozoa None, exhibit a slight Plasmodium.Spp
Amoeboid movement
4. Ciliates Cilia Balantidium coli 13
(2) HELIMINTHS:
The heliminthic parasites are multicellular, bilaterally symmetrical animals having three
germ layers. The helminthes of importance to human beings are divided into three
main groups with the peculiarities of the different categories described in table 2.
TABLE 2. DIFFERENTIATING FEATURES OF HELMINTHES
CESTODE TREMATODE NEMATODE
Shape Tape like, segmented Leaf like, Unsegmented Elongated,
Cylindrical
Sexes Not separate Not separate Separate.
(monoecious) (monoecious) (diecious)
Except blood flukes
which are dioecious
"Head" End Suckers: with hooks Suckers: no hooks No suckers, and
hooks
Alimentary Absent Present Present and
canal but incomplete complete
Body cavity Absent Absent Present 14
(3) ARTHROPODS
Arthropods, which form the largest group of species in the animal kingdom, are
characterized by having a bilaterally symmetrical and segmented body with jointed
appendages. They have a hard exoskeleton, which helps enclose and protect the
muscles and other organs. An open circulatory system, with or without a dorsally
situated heart pumps the blood (hemolymph) via arteries to the various organs and
body tissues. Blood is returned to the heart through body spaces known as
hemocoeles. In addition, respiratory, excretory, and nervous systems are present.
Arthropods affect the health of humans by being either direct agents for disease or
agents for disease transmission.
The arthropods of medical importance are found in Classes Insecta, Arachnida, and
Crustacia which have their own distinguishing features. In Class insecta the body is
divided into head, thorax, and abdomen, with one pair of antennae. Diseases like
malaria, yellow fever, onchocerciasis, and trypanasomiasis are primarily transmitted
by insects.
FIGURE 2. CLASSIFICATION OF ARTHROPODS
Kingdom Animalia
Phylum Arthropoda
Class Class Class Class Class
Crustacia Arachnida Insecta Chilopoda Pentastomida
e.g. Scorpion e.g. Ticks e.g. Mosquito e.g. Centipedes e.g. tongue worms
N.B. Crustacia, Arachnida, and Insecta are the three most common classes of
arthropods of medical significance, which need closer attention15
SUMMARY
A parasite is an organism which lives in/on the body of a host. A host is that which
harbors the parasite. There is usually some association such as mutualism,
commensalisms, or parasitism between the parasite and the host. This association
may produce a variety of effects and the host usually tends to react to it.
Understanding the various structural and behavioral components of parasites assists
classification. In general, the protozoa, helminthes and arthropods are the most
commonly studied and the most important parasites in medical parasitology. They are
further sub classified considering many parameters. 16
REVIEW QUESTIONS
1. Explain briefly the various types of parasites and hosts.
2. Explain the three types of symbiotic relationships and give examples.
3. Discuss the mechanisms by which parasites impose their effect on the host.
4. Give examples of reactions that occur in the body of the host following parasitic
invasion.
REFERENCES:
1. Robert F. Boyd, Basic medical microbiology, third edition, 1986
2. K.D. Chaterjee, protozoology and helminthology, twelfth edition, 1980
3. Brown, H.W. and Neva. F.A. Basic clinical Parasitology (5th edn) New York:
1982.
4. Zaman, V. scanning electron microscopy of medically important parasites.
Littleton, MA: Johnwright PSG, 1983.
5. Belding, D., Text book of clinical parasitology, 2
nd
edition, New York,